In the context of oil and/or gas drilling it is frequently advantageous to detect the flow of fluid into a wellbore. Regardless of whether a flow of gas or liquid into a well is expected, as in the case of production, or unexpected, as in the case of poor formation sealing, information about the location and/or rate of flow can be used to guide subsequent action. Because the environment several thousand feet down in a well tends to be hot, highly pressurized, and variable, many types of sensors that are effective in ambient conditions at the earth's surface are not effective for downhole applications. Hence, it is desirable to provide sensors that can provide accurate fluid flow information downhole and a method for using the same.
Various types of fiber optical systems for measuring mechanical events on the earth's surface are known. For example, U.S. Pat. No. 7,040,390 discloses a security system that uses the intensity and backscattering of optical signals to detect and locate mechanical disturbances to a perimeter border formed of optical cable. Also known are fiber optical sensors for use in downhole flow meters that use strain-sensitive Bragg gratings in a core of one or more optical fibers. The sensors may be combination pressure and temperature (P/T) sensors, such as are described in U.S. Pat. No. 5,892,860, entitled “Multi-Parameter Fiber Optic Sensor For Use In Harsh Environments.” Alternatively, downhole flow measurement systems may use a fiber optic differential pressure sensor or velocity sensors similar to those described in U.S. Pat. No. 6,354,147, entitled “Fluid Parameter Measurement In Pipes Using Acoustic Pressures.”
Similar systems are also disclosed in U.S. Pat. Nos. 7,652,245, 6,414,294, 6396,045, and Application Nos. 2009/0080828 and 2007/0129613, all of which are incorporated herein by reference.
In addition, noise logging conducted inside production tubulars is known in the industry and has been used for the determination of fluid flow in wells for both inflow and outflow (injection) settings with gas and liquids. A noise log is a record of the sound, produced by fluid flow, measured by a microphone at different positions in the borehole. The log may be either a continuous record against depth or a series of stationary readings. Analysis correlating flow-rates to amplitude of recorded noise at various frequencies is well established for conventional microphone devices. Nonetheless, problems with the existing technology as applied to flow measurement across the full well life cycle, from hydraulic fracture stimulations through production operations, include:                the acquisition of this information requires a well intervention activity and gathers data over a limited time interval; acquiring data over full life cycle of the well would be operationally expensive and impractical;        to achieve near continuous coverage over the entire wellbore, an impractically large number of microphones would need to be deployed;        the existing noise logging technique is unable to acquire data beneath wellbore obstructions, such as bridge plugs;        conducting the measurement in a horizontal well, for example, is operationally complex, presents mechanical risks, and is costly;        to match the frequency range provided by this invention would require the use of multiple microphones with a range of frequencies;        long term reliability of the tools for continuous use would be an issue;        the introduction of the logging tool, by its presence in the flow conduit, can change the flowing conditions of the well when conducting measurements and can be an unwanted flow restriction during operations, especially during hydraulic fracture stimulation activities; and        the wireline cable and logging tool for noise-logging are unlikely to effectively operate in the harsh downhole environment during hydraulic fracture or acid stimulation. The stimulation fluids can for example contain high proppant concentrations which will lead to erosion or the injection fluid contains acid or CO2 which will yield corrosion. This will cause in-wellbore equipment to fail during these operations.        
On the other hand, installing microphones outside the production tubulars presents the following problems:                the microphones need to be sufficiently robust to survive the installation process of running the tubulars into harsh subsurface environment (including possible cementing operations);        to provide near continuous on depth coverage and the broadband frequencies would require that an impractically large number of microphones and cables be installed which would complicate installation activities;        the microphones would need to be sufficiently robust to survive the elevated pressures associated with hydraulic fracture stimulation as well, while maintaining the sensitivity needed for behind conduit measurement; and        microphones would be required to have high reliability over the full life of the well, which is not practically available.        
Thus, despite the advances that have been made, it remains desirable to provide a low-cost, system that is robust and easy to install and operate, and that provides accurate flow information downhole.
In particular, Optical Time-Domain Reflectometry (OTDR) techniques for detecting acoustic disturbances, with conventional telecom optical fibers as the sensing element are well known in the security and surveillance business. OTDR techniques with optical fibers for detecting leaks from pipelines are also known. One problem with applying these techniques downhole is that the existing technologies are useful for detecting a flow point but they have not been calibrated to the degree necessary to provide quantification of flowrates, flowregimes, fluid compositions, or changing conditions of the flow point in this setting, and they have not been calibrated for axial flow quantification.